Did 214 in a wet and horrible 12 hour road TT, found it much easier than a 6hr MTB race, still going strong at the finish. The step up to 24 was huge though, really struggled at times, fell asleep twice (once on the bike and once not). That was even wetter and more horrible, bit disapointed to only do 316, but looking at the amount of people that DNF'd I don't suppose it was too bad.
Advicce? Eat loads, even when you're not hungry. Work out how much you think you will eat and take double that. Also have bottles at all times, one energy drink and one plain water. Set of dry clothes every 6-7hrs really helps too, even if just socks/gloves/base layer.